MB-11822 correct bucket low priority to be <= 5

This is so that it conforms to the expectations in the 3.0
upgrade documentation.

Change-Id: I020dc1a8af097362e596c5808cad623e1cf05ab3
Reviewed-on: http://review.couchbase.org/39891
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>